Many subsurface enclosed spaces require periodic inspection to evaluate the condition of the enclosure walls and the soil conditions surrounding the enclosure. Obvious examples are water and sewer pipe systems, but other examples of subsurface enclosures may include military bunkers, underground storage structures, subway tunnels, tunnels in large infrastructure (e.g., dams) or vertical enclosures such as missile silos or water cisterns. Underground enclosures deteriorate over time under the action of various applied and environmental loads, chemical and microbiological induced corrosions and differential settlements. Many underground pipes across the US average 60 years of age and have exceeded their designed life expectancy. The failure of the pipe system may allow surrounding soil and other materials to be eroded and washed into the pipe (infiltration) causing a large void in the soil surrounding the pipe. If this condition becomes sufficiently acute, it can lead to catastrophic surface subsidence, commonly known as “sinkholes.” If the void and subsidence occur beneath a roadway or building, the damage to infrastructure is severe and can have life-threatening consequences.
Sufficiently large subsurface pipelines such as sewer lines and storm water drains are often inspected using manual access. This involves a person entering the pipeline system and carrying out a manual inspection of the pipe walls. However, this method reveals only surface defects in the pipe walls and gives no information on defects in the surrounding soils. This method also has inherent dangers and health risks for the person carrying out the inspection.
More recently, remote controlled Closed Circuit Television (CCTV) has also been employed in inspecting subsurface pipelines. This method involves the use of a small camera which is mounted on a sledge at the end of a flexible cable or on a self-propelled transporter. The camera is placed into the pipeline through an access opening and is then remotely controlled from the surface. This method removes the dangers involved in a person entering the pipeline, however, this method again reveals only surface defects in the pipe walls and gives no information on defects in the surrounding soils.
In order to inspect the bedding of a subsurface pipeline as well as detect defects that cannot be easily identified using CCTV equipment, more elaborate techniques have been employed such as ground probing radar (GPR) and sonar, profilometers, and seismic methods. Sonar systems are typically used to detect deposits at the invert of the pipe below the water level. Profilometer systems are typically used to measure the ovality of the pipe. The GPR systems normally consist of a transmitting antenna emitting electromagnetic radiation, a receiving antenna and an energy detecting device, or receiver. A portion of the transmitted signal is intercepted by a reflecting object, such as the wall of the pipeline, and is reradiated in all directions. The energy reradiated in the back direction is collected by the receiving antenna and delivered to a receiver, where it is processed to detect the presence of the pipeline. The time taken for the radar signal to traverse through the pipe and back is measured. Defects in the soil surrounding the pipeline are detected by using time measurement and known soil characteristics, and comparing this information to site drawings.
Seismic methods measure the velocity and refraction of seismic waves in a manner similar to the electromagnetic radiation measurements of GPR. However, seismic methods are based on long wavelengths with a resulting reduction in resolution. Further, both the GPR and seismic methods require complex equipment and processing which results in low productivity and high costs.